1. Field of the Invention
This invention relates to a prosthesis for replacement of arthritic or damaged elbow joints. More particularly, it concerns a form of elbow joint prosthesis that has good joint stability with minimum bone removal while providing normal flexion and extension motion; there is no in-built constraint and little strain on the seating of the prosthesis.
2. Description of the Prior Art
In general, prosthetic replacement of the elbow has proved a difficult and often disappointing task over the years and this has been the experience of most surgeons in this field. In the past, it was common to replace the elbow joint by means of a constrained prosthesis, usually consisting of a large stem inserted into the humerus and another into the ulna and the two parts of the component linked together by an axle pin. The strong forces on the elbow tend to disrupt the prosthesis from the bone and a very great deal of bone had to be removed to put in such a prosthesis, resulting in the long-term in a disastrous situation very often if the prosthesis has, in fact, to be removed.
There has been a move in recent times to surface replacement of the elbow joint and my work in this area led to the development of a constrained device with a T-slot stability factor which is described in U.S. Pat. No. 4,079,469. Numerous other patents have issued describing bone joint prostheses of which the following is a representative listing:
U.S. Pat. Nos. 2,784,416; 3,547,115; 3,748,662; 3,798,679; 3,801,990; 3,816,854; 3,840,905; 3,852,831; 3,869,729; 3,886,599; 3,919,725; 3,990,116; 4,038,704; 4,057,858; and United Kingdom Pat. No. 1,444,724.
U.S. Pat. No. 3,852,831 describes an endoprosthetic elbow joint having an ulnar component with a saddle shaped bearing surface and a humeral component including a bifurcated member supporting a bearing member between the free ends of its arms. The humeral bearing member has a bearing surface which is arranged to engage in and across the saddle shaping of the ulnar component for mutual articulation. In a preferred arrangement the ulnar component includes a platform with which a bearing member is releasably connected suitably by a sliding dovetail-section keying, the connecting interfaces are circularly curved, and the bearing member can pass through the humeral component. With this arrangement the humeral component and ulnar platform are fixed in place and the ulnar bearing member is then passed through the humeral component in bearing engagement therewith to connect with the platform.
U.S. Pat. No.
4,038,704 discloses an elbow prosthesis comprising a humeral component and an ulnar component, in which each component comprises a joint portion for forming the joint of the prosthesis and an implant portion for implantation into the bone, the joint portion of the humeral component being capable of being received by the joint portion of the ulnar component in snap-fit engagement, while permitting the humeral component to pivot relative to the ulnar component. In order to achieve the snap-fit engagement between the humeral component and the ulnar component, the articular surface of the ulnar component subtends more than 180.degree. at the axis of turning of the prosthesis.
U.S. Pat. No. 4,057,858 discloses an elbow prosthesis including humeral and ulnar components in which the head of the humeral component has a convex medial condylar formation presenting a smooth curved trochlear groove in the shape of part of a helix and providing a bearing surface for a head of the ulnar component and in which the ulnar component presents a condylar head contoured to fit into the trochlear groove, so that as the ulnar component turns around the humeral component, with the condylar head mating with the trochlear groove, the ulnar component moves along the pivotal axis and the ulnar performs the required valgus in extension and varus in flexion. Thus, the humeral articulation groove is in the shape of part of a helix, in order that the elbow adopts a valgus position in extension and a varus position in flexion. However, patients vary in that some have a valgus angle in extension of the elbow, some, but fewer, have a varus angle in extension, and others are between the two, namely having substantially zero angle in extension. This prosthesis, in being designed for a valgus angle in extension cannot cope with the whole range, without serious risk of displacement.
U.S. Pat. No. 3,547,115 describes an osteoarticular prosthesis and prosthetic method which is particularly adapted for use on the distal humerus. The prosthesis has an outer articular surface corresponding to the articular surface being replaced. The bone upon which the prosthesis is to be mounted is preferably first trimmed to fit a keyhole-type opening in the prosthesis. The prosthesis has a sharpened leading edge and is transversely driven onto the bone. Once in place, the prosthesis is locked against displacement by its keyhole-type interengagement with the bone. Consequently, the articular surface of the prosthesis includes a part-spherical capitellum. Naturally, the maximum radius of the capitellum relative to the axis of the joint is determined by the radius bone or radial component with which the capitellum co-operates. Since the articular surface of the humeral component co-operating with the ulna or ulnar component terminates at one axial end adjacent to this part-spherical capitellum, the radius of that one axial end is less than the maximum radius of the capitellum. Therefore, this one axial end presents a low obstruction to lateral displacement of the ulna or ulnar component relative to the humeral component in the direction of the capitellum.
Similar comments apply in respect of U.S. Pat. No. 3,919,725, which discloses an endoprosthetic elbow joint comprising humeral and ulnar components which are each of trough form. The humeral component outer surface is of circular cylindrical form over at least part of its length and is secured to the humerus at its inner surface, while the ulnar component has a complementary circular cylindrical inner surface for mutually articulatory bearing engagement with the humeral component and is secured to the ulna at its outer surface. In a modification the humeral component has a convex, part-annular, part-spherically shaped extension to its outer surface, which extension articulates with a complementary concave surface of an additional component for securement to the radius.
Various ways are known of fixing a plastics prosthetic articular head to a metal prosthetic stem. For example, U.S. Pat. No. 3,816,854 dislcoses a humeral component consisting of a generally cylindrical head of ultra high density polyethylene and a chrome-nickel-cobalt alloy stem incorporating at its distal end a hollow partial cylinder which closely embraces the head. Shallow protruberances on the inner surface of the partial cylinder engage in shallow depressions in the outer surface of the head to retain the head on the stem. U.S. Pat. No. 4,131,956 describes a humeral component consisting of a generally cylindrical internally threaded head of a biocompatible plastics, a forked stem of biocompatible metal, and two screws screwed into the threaded interior of the head through the respective limbs of the fork of the stem.
In spite of the numerous procedures and devices previously developed and used for elbow reconstruction, there exists a need for further improvement particularly as regards resulting joint stability, minimal removal of bone for insertion and allowance for normal flexion and extension motion as well as minimal strain on the seating of the prosthesis.